Kostromateuthis Roemeri Gen

A rare coleoid mollusc from the Upper Jurassic of Central Russia LARISA A. DOGUZHAEVA Doguzhaeva, L.A. 2000. Arare coleoid mollusc from the Upper Jurassic of Central Rus- sia. -Acta Palaeontologica Polonica 45,4,389-406. , The shell of the coleoid cephalopod mollusc Kostromateuthis roemeri gen. et sp. n. from the lower Kirnmeridgian of Central Russia consists of the slowly expanding orthoconic phragmocone and aragonitic sheath with a rugged surface, a weakly developed postalveolar part and a long, strong, probably dorsal groove. The sheath lacks concentric structure common for belemnoid rostra. It is formedby spherulites consisting of the needle-like crystallites, and is characterized by strong porosity and high content of originally organic matter. Each spherulite has a porous central part, a solid periphery and an organic cover. Tubular structures with a wall formed by the needlelike crystallites are present in the sheath. For comparison the shell ultrastructure in Recent Spirula and Sepia, as well as in the Eocene Belemnosis were studied with SEM. Based on gross morphology and sheath ultrastructure K. memeri is tentatively assigned to Spirulida and a monotypic family Kostromateuthidae nov. is erected for it. The Mesozoic evolution of spirulids is discussed. Key words : Cephalopoda, Coleoidea, Spirulida, shell ultrastructure, Upper Jurassic, Central Russia. krisa A. Doguzhaeva [[email protected]], Paleontological Institute of the Russian Acad- emy of Sciences, Profsoyuznaya 123, 117647 Moscow, Russia. Introduction The mainly soft-bodied coleoids (with the exception of the rostrum-bearing belemnoids) are not well-represented in the fossil record of extinct cephalopods that results in scanty knowledge of the evolutionary history of Recent coleoids and the rudimen- tary understanding of higher-level phylogenetic relationships of them (Bonnaud et al. 1997;Carlini & Graves 1999). Modern corntnonly accepted classification (Sweeney & Roper 1998) splits the coleoids into two superorders, the Decabranchia including the orders Spirulida, Sepiida, Sepiolida and Teuthida, and the Octobranchia including the orders Octopodida and Vampyromorphida. Because of its shell a single extant spirulid Spirula spirula has great potentialities for the evolutionary morphology of cephalopods. Voss (1977) considered this form as a member of the order Sepioidea, but later it was demonstrated with the aid of the mo- Acta Palaeontol. Pol. 45,4,389-406. 390 Jurassic coleoid mollusc: DOGUZHAEVA lecular phylogenetic analysis that Sp. spirula is linked neither to sepioids nor to teuthoids (Bonnaud et al. 1997; Carlini & Graves 1999). The evolution of spimlids is obscure, thus any shell found which might have belonged to this branch of coleoid cephalopods is of exceptional interest. The Jurassic record of spirulids included a single Oxfordian form Plagioteuthis moscoviensis Roemer, 1890 (see Biilow-Trumrner 1920). It is known by a sole rostrum-llke structure with a deep alveolus and had no indications of conotheca, septa or siphuncle. Because of this the taxonomic assignment of the genus remains doubtful (Doyle et al. 1994). The present paper deals with a better preserved shell belonging to a previously unknown Early Kimmeridgian phragmocone-bearing orthoconic coleoid with a sheath-like structure instead of a real rostrum. It is described herein as Kostro- mateuthis roemeri gen. et sp. n. The sheath ultrastructure of K. roemeri studied with SEM yielded much new information concerning the ultrastructure of the coleoid shell that is useful for the comparison of K. roemeri, spimlids, sepiids, and belemnoids. Materials and methods The description of Kostromateuthis roemeri is based on a shell from the lower Kimmeridgian clay beds in the basin of the River Unzha, on its right bank near village Popovo, Kostroma Region, Central Russia. It was found together with several ammonites by a private collector, Mr. A.V. Stupachenko in 1998. V.V. Mitta (personal com- munication) assigned the ammonites to the following species: Amoeboceras (Amoe- bites) ex. gr. kitchini, A. (A.) pulchrum, Prorasenia aff. stephanoides, and Rasenio- ides? sp. juv. The ammonoid assemblage indicates the standard Cymodoce Zone of the lower Kimmeridgian (Hantzpergue et al. 1998). The specimen was studied with the scanning electron microscope (SEM) in the PaleontologicalInstitute, Moscow. The micrographs were taken from the side showing the longitudinal groove and of the conical apical portion of the sheath; the latter being studied in fractured cross surface and from its outer surface. For comparison, the ultrastructure of the sheath of Recent Spirula from Cuba, Sepia from Australia, and Belemnosis from the Eocene of Turkey was studied with the SEM. The shells were etched with 5 per cent hydrochloric acid for 3-5 seconds. The studied material is deposited in the Paleontological Institute of the Russian Academy of Sciences, Moscow, abbreviated as PIN. Description Class Cephalopoda Cuvier, 1794 Subclass Coleoidea Bather, 1888 Superorder Decabranchia Boettger, 1952 ?Order Spirulida Pompeckj, 1912 Family Kostromateuthidae nov. Type genus by monotypy: Kostromateuthis nov. ACTA PALAEONTOLOGICA POLONICA (45) (4) 391 Diagnosis. -Small (approximately 24cm in length) aragonitic orthocones, slowly expanding with bilaterally symmetrical sheath instead of rostrum. Sheath posteriorly conical; postalveolar portion absent or short; surface uneven; consisting of spherulites with much organic matter; thin (presumably) ventrally, thickened dorsally with the longitudinal central narrow part, which is thicker than elsewhere, bearing a groove, dorsally. The groove long, deep, bordered by two reinforced ridges with rows of tubercles along their crests and few smaller ridges on its floor. Phragmocone with simple camerae and straight septa; mural parts of septa long (about 113-215 of camera length); sutures non-sinuous. Occurrence. -Upper Jurassic, lower Kimmeridgian; Unzha River, Kostroma Region, Central Russia. Remarks. -The groove on the sheath is an apomorphy of the new family and distinguishes it from the older (Stephanian) family Shimanskyidae Doguzhaeva, Mapes, & Mutvei, 1998 and younger (Cretaceous) families Adygeyidae Doguzhaeva, 1996 and Groenlandibelidae Jeletzky, 1966, which have smooth shells, lacking grooves. The family is tentatively referred to the Order Spirulida on the basis of the following common features: well developed slowly expanding phragrnocone with comparatively short camerae, simple sutures, and septa with long mural parts; presence of a sheath with uneven surface and spherulitic ultrastmcture instead of a real rostrum. The protoconch structure, namely: whether it had a closing membrane as in belemnoids, or a calcified first septum with a septa1 foramen, a caecum and a prosiphon as in spirulids, is so far unknown. But the family can hardly be referred to belemnoids because of the dimpled surface of the sheath which is a resull or ils spherulitic structure. Among belemnoids, the new family resembles diplobelids in having no postalveolar portion of the rostrum. However, diplobelids have breviconic phragmocones, sinuous sutures and crowded septae (Jeletzky 1981: p. 115), unlike the new family. The reconstruction of the shell orientation is based on the fact that in coleoids the ventral side of the shell gradually lost its protective function and was replaced by a muscular mantle wall. For this reason the thickened side that bears the groove is considered as dorsal. Genus Kostromateuthis nov. Type species by monotypy: Kostromateuthis roemen' gen. et sp. n. Derivation of name: From Kostroma, the capital of the region where the specimen was found. Diagnosis. - Miniature orthocones with angle of expansion 18-20 degrees. Sheath conical with rounded apex and irregular rows of bean-shaped depressions apically, and smoother, 'pock-marked' adorally. Groove bordered by two ridges with rows of more than 10 tubercles along their crests; two or three smaller ridges on its floor. Remarks. -In the uneven outer surface the sheath of Kostromateuthis is similar to partitions of the sheath of the Tertiary spirulid Beloptera (Dauphin k985: fig. 3f) and the outer surface of the sheath of Belemnosis. Also the tubercles along the crests flanking the groove show similarity to the humps on the outer surface of a longitudinal partition of the Beloptera sheath (Dauphin 1985: fig. 4a). These were interpreted as unbroken spherulites (Dauphin 1985: p. 330). Spherulites with central depressions are known in the sheath of the Paleocene Belopterina (Dauphin 1986: pl. 2a-c). Kostromateuthis roemeri gen. et sp. n. Fig. 1A-D. Holotype: PIN 38711221 - the only specimen known. Type horizon: Upper Jurassic, lower Kimmeridgian. Type locality: Unzha River, right bank near village Popovo, Kostroma Region, Central Russia. Derivation of name: in honour of Ferdinand Roemer, who described the first Jurassic coleoid shell which might have belonged to the Spirulida. Diagnosis. - Same as for genus. 392 Jurassic coleoid mollusc: DOGUZHAEVA ACTA PALAEONTOLOGICA POLONICA (45) (4) 393 Description. - The shell is small (24 mm long, with maximum diameter about 8 mm), orthoconic, slowly expanding (angle of expansion 18-20 degrees), surface uneven, especially in its apical part where it is reticulated. Phragmocone and sheath seem to be of equal length; sheath probably ends near the apertural margin of the phragmocone and does not extend beyond the phragmocone. The sheath bears a strong medial, presumably dorsal, groove bordered by two ridges

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